U.S. patent application number 14/240932 was filed with the patent office on 2014-07-24 for device for stable subsea electric power transmission to run subsea high speed motors or other subsea loads.
This patent application is currently assigned to Aker Subsea AS. The applicant listed for this patent is Kjell Olav Stinessen. Invention is credited to Kjell Olav Stinessen.
Application Number | 20140203640 14/240932 |
Document ID | / |
Family ID | 47883516 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140203640 |
Kind Code |
A1 |
Stinessen; Kjell Olav |
July 24, 2014 |
DEVICE FOR STABLE SUBSEA ELECTRIC POWER TRANSMISSION TO RUN SUBSEA
HIGH SPEED MOTORS OR OTHER SUBSEA LOADS
Abstract
The invention provides a device for operative connection between
a subsea step out cable far end and subsea loads such as pumps,
compressors and control systems, distinctive in that the device is
a rotating frequency stepper device, more specifically a rotating
step up or step down device, and it comprises: a motor and a
generator operatively connected so that the motor drives the
generator, at least one gas and/or liquid filled vessel into which
at least one of the motor and generator are arranged, and the step
out length is long, which means long enough to cause problems due
to the Ferranti effect at frequency and power levels feasible for
subsea pump and compressor motors, and where the device via the
step out cable receives input electrical power at a low enough
frequency to have stable transmission and the device, operatively
connected to the subsea motor, delivers an output electrical
frequency, amperage and voltage feasible for operation of the
connected motors. System for pressure boosting of hydrocarbon fluid
or other fluid subsea, comprising the device.
Inventors: |
Stinessen; Kjell Olav;
(Oslo, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stinessen; Kjell Olav |
Oslo |
|
NO |
|
|
Assignee: |
Aker Subsea AS
Lysaker
NO
|
Family ID: |
47883516 |
Appl. No.: |
14/240932 |
Filed: |
September 11, 2012 |
PCT Filed: |
September 11, 2012 |
PCT NO: |
PCT/NO2012/050174 |
371 Date: |
February 25, 2014 |
Current U.S.
Class: |
307/31 |
Current CPC
Class: |
H02M 5/32 20130101; H02J
3/22 20130101; H02P 5/74 20130101; Y02E 10/30 20130101 |
Class at
Publication: |
307/31 |
International
Class: |
H02J 3/22 20060101
H02J003/22; H02M 5/32 20060101 H02M005/32 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 2011 |
NO |
20111233 |
Claims
1. A device for operative connection between a subsea step out
cable far end and subsea loads such as pumps, compressors and
control systems, characterised in that wherein the device is a
rotating frequency stepper device, the device comprising: a motor
and a generator operatively connected so that the motor drives the
generator; at least one gas and/or liquid filled vessel into which
at least one of the motor and generator are arranged; wherein the
step out length long enough to cause problems due to the Ferranti
effect at frequency and power levels feasible for subsea pump and
compressor motors and wherein the device via the step out cable
receives input electrical power at a low enough frequency to have
stable transmission and the device, operatively connected to the
subsea motor, delivers an output electrical frequency, amperage and
voltage feasible for operation of the connected motors.
2. The device according to claim 1, wherein the device has no means
for active control or adjustment on site subsea, and the device
comprises: a generator arranged on a motor shaft; a vessel into
which the motor, generator and shaft are arranged; a liquid filling
the vessel; a pressure compensator; and at least one electric
penetrator.
3. The device according to claim 1, comprising an electric motor
and an electric generator having a common shaft, the pole number of
the generator is a multiple of the pole number of the motor, and
the number of poles of the motor and generator is selected such
that the desired frequency step-up is achieved.
4. The device according to claim 1, wherein the passive electric
frequency stepper device comprises one of: a mechanical gear, a
fluid-dynamic or hydraulic gear, a mechanical fluid dynamic gear or
a magnetic gear.
5. The device according to claim 1, wherein the connection or
coupling or shaft comprises a hydraulic or fluid coupling.
6. The device according to claim 5, wherein the coupling is a soft
starter.
7. The device according to claim 1, wherein a VSD is connected at
the near end to adjust the low frequency transmission frequency and
thereby adjust the output frequency of the generator up and down to
give the desired speed of the connected motor or motors.
8. The device according to claim 1, wherein the transmission
frequency from the power source at the near end is fixed.
9. The device according to claim 1, wherein a housing is filled
with liquid, preferably oil or a mixture of water and antifreeze
agent and have a pressure balancing device between the ambient
seawater and the internal liquid of the housing.
10. The device according to claim 1, comprising two or more SRFSDs,
wherein the motors of the SRFSDs are connected to bundled
transmission lines.
11. The device according to claim 1, wherein the motor of the
subsea RFSD is of the high voltage type with insulated cables in
the stator.
12. The device according to claim 1, wherein: the housing is filled
with gas; and the pressure inside the housing can be selected from
in the region of one bar up to equal to the ambient water pressure
or higher.
13. The device according to claim 1, wherein the generator is a DC
generator.
14. A system for pressure boosting of hydrocarbon fluid or other
fluid subsea, the system comprising: a subsea step out cable,
connected to a an electric AC power source at a near end, the
length of the subsea step out cable is too long for stable
operation at frequency and power level feasible for subsea pressure
boosting equipment; subsea motors for pumps or compressors
operatively connected to a far end of the subsea step out cable;
and a rotating motor-generator frequency step up device arranged
between the subsea step out cable and subsea pumps or
compressors.
15. The system according to claim 14, wherein the device has no
means for active control or adjustment on site subsea, and the
device comprises: a generator arranged on a motor shaft; a vessel
into which the motor, generator and shaft are arranged; a liquid
filling the vessel; a pressure compensator; and at least one
electric penetrator.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to equipment for subsea
production of petroleum, particularly equipment located far away
from dry topside or onshore locations. More specifically, the
invention relates to equipment for electric power transmission to
subsea loads that can be located far away from surface platforms or
shore and require high power transmission. Said loads are typically
motors for pumps and compressors which require control of
rotational speed by control of the electric frequency.
[0002] The invention come to grips with the problems caused by the
Ferranti effect and the skin effect, thereby opening up for longer
subsea step out lengths than previously achievable.
BACKGROUND OF THE INVENTION AND PRIOR ART
[0003] Over the last decades global energy consumption has
increased exponentially and no end can be seen for the increased
demand. Whereas exploitation of fossil fuels was previously focused
on onshore fields, the limited amount of oil started serious
efforts to find and exploit offshore gas and oil fields. Presently
the state of the art for production from offshore fields is by use
of fixed or floating manned platforms, and by tie-in of subsea
production templates with subsea wells to these platforms. In some
cases production is routed directly to an onshore receiving
facility without a platform. In order to maintain a sufficiently
high production from subsea satellites to a central platform or
directly to shore, pressure boosting can be provided by using a
multiphase pump or by separation followed by pumping and
compression. Pumps have also been installed at seabed for direct
seawater injection into the reservoir for pressure support for
enhanced oil production. [0004] There are several advantages that
motivate for subsea location of pumps and compressor stations
compared to location on platforms: [0005] Safety for people by not
working and living on platform and not being transported by
helicopters to and from [0006] No risk of fire and explosion [0007]
No risk for blow-out from production risers up from seabed to
platform and from platform to seabed [0008] Security against
sabotage [0009] Cost saving both for capital and operation, i.e.
reduced production cost for oil and gas [0010] Increased production
because the suction effect of compressors and pumps is closer to
the wellheads [0011] The equipment has stable ambient conditions,
i.e. almost constant, cold temperature and almost constant, low
flow seawater current velocity around the equipment and no waves,
while the temperature at platforms can vary from e.g. -20.degree.
C. to +30.degree. C. and the wind velocity can be at hurricane
strength combined with extremely high waves. [0012] The cold
seawater can be utilized for cooling of motors and other electric
and electronic equipment and process fluids [0013] No visual
pollution [0014] Considerably lower weight and thereby lower
material and energy amount for fabrication of a subsea plant [0015]
Lower carbon dioxide, i.e. climate gas emission for fabrication due
to less material amount [0016] Less carbondioxide emissions during
operation due to elimination of helicopter transport and operation
of platform [0017] Less carbondioxide emission compared to
platforms due to electric motors for running compressors and pumps
and supply of electric power from shore or platform [0018] Less
energy consumption and climate gas emission per weight unit of oil
and gas
[0019] The disadvantage for subsea compressors per 2010 is that
none has been installed and operated subsea, i.e. the technology is
not proven. However, this is just a question of time, and the first
subsea compressor station will probably be in operation in 2015 or
earlier due to the strong motivation for this application.
[0020] Subsea pressure boosting is a recent technology. Subsea
pressure boosting requiring a significant subsea step out length is
a very recent technology using modern equipment and facing problems
that are not met or is irrelevant elsewhere.
[0021] State of the art technology is defined in patent publication
WO 2009/015670 prescribing use of a first converter arrangement in
the near end, the topsides or onshore end, of a subsea step out
cable and a second converter arrangement in the far end, the subsea
remote end, of the subsea step out cable. A variable speed drive,
VSD, is prescribed in either end of the step out cable. Subsea
variable speed drives (VSD) for electric motors is also called
variable frequency drive (VFD) or Adjustable Frequency Drive (AFD)
or frequency converters or just converters and they represents
state of the art technology. Neither in WO 2009/015670 or other
publications is the Ferranti effect mentioned, nor is any problems
associated with subsea VSDs discussed or indicated.
[0022] So far only a few subsea pumps and no subsea compressors are
in operation. Subsea compression stations are however being
developed and the first expected to be installed and in operation
within some few years. Currently, subsea pumps and compressors are
all driven by asynchronous motors. The step-out distance of
installed pumps is not more that about 30 km from platform or shore
and so far the depths are not below 1800 m. It is known that
serious studies and projects are conducted by the oil industry
aiming at installation of compressors at a step-out distance in the
range of 40 to 150 km and at water depth down to 3000 m or
more.
[0023] A realistic motor power is from about 200 kW for small pumps
and up to 15 MW for compressors and in the future even larger
motors can be foreseen. Subsea motors that are presently installed
are supplied with power via AC (alternating current) cables from
the location of the power supply, i.e. platform or shore, and in
case of several motors each motor has its own cable and frequency
converter (Variable Speed Drive, VSD) at the near end of the cable
in order to control the speed of each individual motor at the far
end of the cable, ref. FIG. 1 and Table 2.
[0024] In the context of this patent description near end means the
end of the power transmission near to the power supply. In subsea
applications this is topsides platform location or onshore.
Correspondingly, the far end refers to the other end of the
transmission line close to the power loads, typically motor loads.
Far end is not necessarily restricted to the high-voltage end of
the transmission line. The term can be extended to busses or
terminals of lower voltage which are part of the far end station
such as e.g. a common subsea bus on the low-voltage side of a
subsea transformer.
[0025] Compressors and pumps are often operated at maximum speeds
between 4000 to 14000 rpm and 2000 to 5000 rpm, respectively. Thus
the driving electrical motor has to have a rated speed in the order
2000 to 14000 rpm when using modern high speed motors without a
gearbox between the motor and the pump or compressor. This
mechanical speed corresponds to an electrical frequency range for
the feeding drive of about 30 to 230 Hz for the example of a
two-pole motor. Motors with more pole pairs would allow for lower
maximum mechanical speed for the same electrical frequencies.
[0026] FIG. 1 illustrates the only solution so far used for
transmission of electric power to installed pumps, in some cases
without transformers between VSD and subsea motors, and this is
referred to as First solution. This solution with one transmission
cable per motor has the disadvantage of becoming expensive for long
step-out; say more than 50 km, due to high cable cost.
[0027] A serious technical obstacle against this solution is that
at a certain subsea step-out length, the transmission of electric
power from a near end power source to a far end distant motor is
not feasible because the transmission system will become
electrically unstable and inoperable due to the Ferranti effect
that later will be described. The innovation will resolve this
problem of instability.
[0028] FIG. 2 illustrates a solution that has been proposed for
transmission of electric power to several loads at long step-out,
Solution Two. This solution with one common transmission cable and
a subsea power distribution system including one subsea VSD
(Variable Speed Drive) per motor, will considerably reduce the
cable cost for transmission, and also prevent the problem of
electric instability by limiting the frequency of the current in
the transmission cable to say 50-10 Hz, and the skin effect is also
acceptable for such frequencies. The frequency is then increased by
a VSD to suit the speed of the motor connected to the VSD. The
Second Solution has however also disadvantages. These are expensive
VSDs which are not proven for subsea use, and because such VSDs are
composed of many electric and electronic components included a
control system, they are susceptible to contribute to an increased
failure rate of the electric transmission and subsea distribution
system.
[0029] In the following will be described the inherent electrical
problems of the existing First Solution (FIG. 1), with one motor at
the far end of a long cable, and a Third Solution illustrated in
FIG. 3 with several motors at the far end of a common long
transmission and a common VSD at the near end.
[0030] For a long step-out distance from the power supply to the
load, in the order of 50 km and above, the influence of the subsea
cable is so strong that such a system has not been built yet for a
limited load such as a single motor. The line inductance and
resistance involve a large voltage drop from the power supply to
the load. It is known that such a voltage drop is self-amplifying
and can result in zero voltage at the far end. The longer the
step-out distance the higher the transmission voltage has to be in
order to reduce the voltage drop along the transmission line.
However, a cable has a high capacitance and a long AC (alternating
current) cable will exhibit significant so-called Ferranti effect.
The Ferranti effect is a known phenomenon where the capacitive
charging current of the line or cable increases with the line
length and the voltage level. At a step-out length of 100 km the
charging current in a cable can be higher than the load current,
which makes it difficult to justify such an ineffective
transmission system. A more critical result is that the sudden
no-load voltage will be about 50% higher than the near end supply.
Such highvoltage would destroy the cable and the far end
transformer and connections. At a sudden load drop the far end
voltage will jump to this high level. In addition there will be a
transient peak of e.g. 50% giving like 100% in total, see Table 1
below where values marked with fat italic letters are above the
voltage class margin of the insulation.
[0031] Today's systems with step-out distances in the order 30 km
have not this problem, because the subsea step-out length and
electric load in combination is still feasible.
TABLE-US-00001 TABLE 1 Voltage rise at load trips due to Ferranti
effect in different systems Max. Far end transient Far transmission
frequency f.sub.max and Step- Source Full-load voltage peak u.sub.p
after end shaft power motor speed .omega..sub.max out length
Standard cable voltage at near end U and no-load voltage U
full-load trip Pump 60 Hz 40 km 95 mm.sup.2 20 kV 18.3 kV 20.9 kV
2.5 MW (3600 rpm) 30(36) kV 20.2 kV First Solution Compressor 180
Hz 40 km 150 mm.sup.2 32 kV 29.2 kV 7.5 MW (10800 rpm) 30(36) kV
34.8 kV First solutions Pump 60 Hz 100 km 150 mm.sup.2 26 kV 23.6
kV 28.9 kV 2.5 MW (3600 rpm) 30(36) kV 27.5 kV First Solution
Compressor 180 Hz 100 km 150 mm.sup.2 28.5 kV 28.8 kV 7.5 MW (10800
rpm) 30(36) kV First Solution Three 180 Hz 100 km 400 mm.sup.2 45.6
kV 45.6 kV compressors and Compressor: 45(54) kV three pumps. 10800
rpm Total 30 MW Pump: Third solution 5400 rpm
[0032] The Ferranti effect and skin effect--some
considerations:
[0033] The Ferranti effect is a rise in voltage occurring at the
far end of a long transmission line, relative to the voltage at the
near end, which occurs when the line is charged but there is a very
light load or the load is disconnected.
[0034] This effect is due to the voltage drop across the line
inductance (due to charging current) being in phase with the
sending end voltages. Therefore both capacitance and inductance are
responsible for producing this phenomenon.
[0035] The Ferranti effect will be more pronounced the longer the
line and the higher the voltage applied. The relative voltage rise
is proportional to the square of the line length.
[0036] Due to high capacitance, the Ferranti effect is much more
pronounced in underground and subsea cables, even in short lengths,
compared to air suspended transmission lines.
[0037] A proposed equation to determine the Ferranti effect for a
given system is:
v.sub.f=v.sub.n(1+.omega..times.C.times.L.times.I.sup.2)
Where:
[0038] v.sub.f=far end voltage v.sub.n=near end voltage
.omega.=2.times.3.14.times.f f=frequency C=line capacitance L=line
inductance I=line length I.sup.2=line length square
[0039] In the literature can also be found other expressions for
the Ferranti effect, but in any cases it is agreed that the effect
increases with transmission frequency, cable capacitance, length of
cable and voltage.
[0040] From the above equation can be concluded that the Ferranti
effect of a long line can be compensated by a suitable reduction of
the electric frequency. This is the reason for the Second Solution
with subsea VSD. The transmission frequency can e.g. be the normal
European frequency of 50 Hz.
[0041] Another benefit with low transmission frequency is a strong
reduction of the electrical skin effect of the transmission cable,
i.e. better utilization of the cross section area of the cable. In
practice transmission of high frequency electricity, say 100 Hz or
more over long distances, say 100 km or more, will become
prohibitive due to the skin effect and the corresponding high
resistance of the cable.
[0042] The influence of Ferranti effect and skin effect has of
course to be calculated from case to case to assess whether they
are acceptable or not for transmission at a given frequency. A
demand exists for providing subsea electric power transmission
systems that are beneficial with respect to the above mentioned
problems.
FIGURES
[0043] The invention is illustrated with figures, of which
[0044] FIGS. 1-3 illustrate prior art embodiments, and
[0045] FIGS. 4-7 illustrate embodiments of the present
invention.
SUMMARY OF THE INVENTION
[0046] The invention provides a device for operative connection
between a subsea step out cable far end and subsea loads such as
pumps, compressors and control systems, distinctive in that the
device is a rotating frequency stepper device, more specifically a
rotating step up or step down device, and it comprises: [0047] a
motor and a generator operatively connected so that the motor
drives the generator, [0048] at least one gas and/or liquid filled
vessel into which at least one of the motor and generator are
arranged, and [0049] the step out length is long, which means long
enough to cause problems due to the Ferranti effect at frequency
and power levels feasible for subsea pump and compressor motors,
and where the device via the step out cable receives input
electrical power at a low enough frequency to have stable
transmission and the device, operatively connected to the subsea
motor, delivers an output electrical frequency, amperage and
voltage feasible for operation of the connected motors.
[0050] The device is preferably a passive frequency step up device
or frequency step down device, having no means for active control
or adjustment on site subsea, and it comprises: a rotatable shaft
having a motor arranged; a generator arranged on the motor shaft or
on a different shaft operatively connected to the motor shaft; a
pressure vessel into which the motor, generator and shafts are
arranged; a gas and/or liquid filling the pressure vessel, at least
one electric penetrator and a pressure compensator if the vessel is
filled with liquid that is to be pressure compensated to the
ambient seawater pressure. The frequency step down can be all the
way down to 0 Hz, the frequency step up can be up to the operating
frequency of the connected loads.
[0051] More preferably the device is a Subsea Rotating Frequency
Step-up Device (SRFSD) comprising an electric motor coupled to a
generator for subsea location at a far end of a subsea step out
cable connected to at least one power source at the step out cable
near end at a dry location onshore or topsides, and the step out
length is long, which means long enough to cause problems due to
the Ferranti effect at frequency and power levels feasible for
subsea pump and compressor motors, and where the device via the
step out cable receives input electrical power at a low enough
frequency to have stable transmission and the device, operatively
connected to the subsea motor, delivers an output electrical
frequency, amperage and voltage feasible for operation of the
connected motors and the device is installed in a pressure vessel
or housing that is filled with liquid or gas.
[0052] Most preferably the device comprises an electric motor and
an electric generator having a common shaft, the pole number of the
generator is a multiple of the pole number of the motor.
Alternatively the device comprises one of: a mechanical gear, a
fluid-dynamic or hydraulic gear, a mechanical fluid dynamic gear or
a magnetic gear.
[0053] No earlier subsea pressure boosting systems has taken into
consideration the Ferranti effect. The earlier system version with
a subsea VSD can therefore be useless for many applications since
the insulation of the step out cable can be damaged by
uncontrollable high voltage at the far end due to the Ferranti
effect. The feature a "passive electric frequency step up or step
down (or stepper) device", relevant for some embodiments, means
that the device shall not and can not be adjusted on site during
operation or any time during the service life of the system, the
device is a passive slave unit, namely a passive frequency step up
device or a passive frequency step down device, contrary to a
subsea VSD. A subsea VSD is very complex, large and expensive, it
is typically about 12 m high, 3 m in diameter and weights about 200
tons. The passive device will to the contrary be much smaller and
simpler, being typically about 6 m long and 2-3 m in diameter,
weighting about 50 ton. The reliability of the device is estimated
to be several times better than for a subsea VSD. This is because a
subsea VSD is very complex, and even though all components are of
top quality the large number of components and the complexity
results in a reduced reliability in practice. The cost of the
device or a system of the invention will be significantly reduced
compared to the state of the art systems having a subsea VSD. The
term other loads comprises power to control systems and other loads
not necessarily related to pressure boosting.
[0054] The operation frequency of the step out cable must be
considered taking into account the Ferranti effect and the
electrical losses. The insulation is a key element. Most
preferably, the dimensions of conductors and insulation, and choice
of operation frequency, are so that at the far end of the cable,
the Ferranti effect, at its maximum during operation, increases the
voltage just as much as the electrical losses, hence overvoltage at
the far end due to the Ferranti effect is avoided and the cable
design is simplified. The guidance provided in this document,
combined with good engineering practice, is assumed to be
sufficient for proper step out cable design, including choice of
operation frequency: The solution should be found in each case. The
device of the invention is then designed in order to transform the
operation frequency of the step up cable to the operation frequency
of the subsea loads, i.e. subsea compressors or pumps, or more
specifically, the motors of the subsea compressors or pumps.
[0055] Further embodiments and features are defined in the
dependent claims. The features described or illustrated in this
document can be included in the device of the invention in any
operative combination, and each such combination is an embodiment
of the invention. The motivation for such combinations is based
upon what is described or illustrated or the combinations are
obvious for persons skilled in the art after having studied this
document thoroughly.
[0056] The input and output electrical frequency of the device will
be different. The difference will be at a fixed ratio for passive
devices. The input frequency, the operation frequency of the step
out cable, will be in the range 0.1-150 Hz, such as 2-60 or 4-50 Hz
or 5-40 Hz, whilst the output frequency will be in the range
0.1-350 Hz, such as 30-300 Hz, 50-250 Hz or 50-200 Hz. The output
frequency can also be 0, i.e. direct current (DC) by using a DC
generator in the motor-generator set. The subsea device can be
arranged in one or several housings, as one or several elements,
however, all parts of it must withstand the harsh subsea
environment without failure. With the present invention, the long
term cost and reliability of said device , and associated systems,
improve significantly over what is currently achievable with for
example subsea solid state variable speed drives.
[0057] The invention also provides a system for pressure boosting
of hydrocarbon fluid or other fluid subsea, comprising [0058] a
subsea step out cable, connected to a an electric AC power source
at a near end, the length of the subsea step out cable is too long
for stable operation at frequency and power level feasible for
subsea pressure boosting equipment, [0059] subsea motors for pumps
or compressors operatively connected to a far end of the subsea
step out cable, [0060] distinctive in that the system further
comprises: a rotating motor-generator frequency step up device
arranged between the subsea step out cable and subsea pumps or
compressors.
[0061] Preferably, the device of the system has no means for active
control or adjustment on site subsea, and it comprises: [0062] a
generator arranged on a motor shaft, [0063] a vessel into which the
motor, generator and shaft are arranged, [0064] a liquid filling
the vessel, [0065] a pressure compensator, and [0066] at least one
electric penetrator.
[0067] In addition, the invention provides use of a subsea rotating
stepper device of the invention for transforming the electrical
power characteristics of a subsea step out cable to an electric
power characteristic feasible for operation of connected subsea
equipment, a system with at least one subsea stepper device of the
invention arranged in the far end of a subsea step out cable, and a
method of operating said system, by control adjustments only for
system items at dry topsides or onshore locations, such as by a
topsides VSD. Either one of the device, the system, the method or
the use of the invention, may comprise any features or steps as
herein described or illustrated, in any operative combination, each
such operative combination is an embodiment of the invention.
The Embodiment of the Invention with Frequency Step-Up to Run AC
Motors
[0068] An embodiment of the invention, the Fourth Solution is shown
in FIGS. 4 and 5. The main feature of the embodiment is
introduction of a subsea frequency step up or step down device, in
the illustrated embodiment a frequency step-up device (FSD) located
subsea at the far end of the transmission cable and at a short
distance to the motors that runs the compressors and pumps. Short
distance means in this context near enough to keep acceptable the
ohmic resistance drop and thereby power loss between the
generator/FSD and the motors, and it also means short enough to
avoid problems caused by Ferranti effect and instability. It is
important to note that the subsea FSDs are not directly controlling
the frequency to suit the operational speed of motors by having a
local control system that adjusts the speed according to needs. The
variation of speed according to steady state production need, start
and stop and ramping speed down and up, is done by the near end
surface (topsides on platform or onshore) located VSD or by other
means far from the subsea FSDs. The FSDs are simply slaves of the
VSD and their purpose is only stepping-up the transmission
frequency given by the VSD by some multiple.
[0069] This step-up is easiest obtained by using a subsea electric
motor which shaft is coupled to a subsea electric generator and
both machines running with same speed, i.e. a subsea rotating FSD
(RFSD). Any type of coupling (e.g. flexible, rigid, common shaft of
motor and generator, hydraulic, fluid coupling) can be used that
gives the same speed of motor and generator. The motor should
preferably have 2-poles to keep the transmission frequency as low
as possible, while the generator's number of poles will be chosen
according to the need for step-up from a transmission frequency
that is low enough to not give the above described problems caused
by Ferranti effect, instability and high resistance due to skin
effect with corresponding unacceptable voltage drop.; i.e. within a
"problem free frequency range".
[0070] By having a 2-pole motor and a 4-pole generator the step-up
ratio will be 2:1, a 6-pole generator will give a ratio of 3:1 and
an 8-pole generator 4:1 and so on dependent of the number of poles
of the generator. This means that if the frequency from a surface
VSD is in the range of 50 the subsea frequency from the subsea RFSD
device will be in the range of 100 Hz corresponding to a
revolutionary speed of 2-pole motors from 6000 rpm. If using an
8-pole generator the corresponding stepped-up frequency will be in
the range of 200 and the speed of a 2-pole motor 12000 rpm. These
examples clearly demonstrate that the invention can supply any
needed frequency for realistic motor speeds by a correct
combination of poles of motor and generator of the rotating subsea
RFSD and at a problem free transmission frequency.
[0071] Generally the step-up ratio can be expressed:
fsu=n.times.ft, where
ft: transmission frequency, Hz fs-u: stepped-up frequency=input
frequency to motors, Hz n: multiple 2, 3, 4 and so on dependent of
number of poles of the generator compared to the motor
[0072] The problem free frequency range must be calculated from
case to case. For step-out distances of up to say 150 km a
transmission frequency of up to say 75 Hz could be within the
problem free range which will give a 2-pole compressor motor speed
of 2.times.7.times.60=9000 rpm if the step-up ratio is 2:1 (2-pole
motor and 4-pole generator). If 75 Hz is found to be to high to be
problem free, a step-up ratio of 3:1 (2-pole motor and 6-pole
generator) can be applied, which for the given example will reduce
the transmission frequency to maximum of 50 Hz. The transmission
frequency will not stay constant over the whole production period
of the oil or gas field, but have to be adjusted up over time as
the pressure at the wellheads decreases. For a given case the
transmission power from the near end could be 33.3 Hz by the
beginning and 50 Hz by the end of production corresponding to a
speed of between 6000 and 9000 rpm of a 2-pole compressor motor at
the far end.
[0073] By selecting the right step-up ratio by selection of poles
of motor and generator, it will probably be possible to transmit AC
power problem free to subsea motors with a distance from the near
end to the far end (step-up distance) of 300 km or more.
[0074] Use of a 2-pole motor is beneficial to keep the transmission
frequency as low as possible. If there of other reasons, e.g.
torque and power, should be found favourable to use a motor with
higher number of poles, it is still possible to get a desired
step-up by selecting the number of poles of the generator
correspondingly, e.g. 4-pole motor and 12-pole generator will give
a step-up ratio of 3:1.
[0075] An advantage by using low frequency and 4-poles motor is
that the speed of the motor and generator will be low and so will
be the corresponding frictional losses in the motor. This opens for
use of oil filled motor and generator arranged in common pressure
housing,
[0076] If for instance the transmission frequency is 25 Hz and a
4-pole motor is used the rotational speed will be only 750 rpm,
which will result in low frictional losses. To achieve a frequency
of 150 Hz from the generator, this has to be 24-pole. By varying
the transmission frequency from 18 to 28 Hz, the frequency from the
generator will vary in the range from 108 to 168 Hz and give motor
speed (2-pole) of 6480 to 10080, which could be suitable for a
compressor motor.
[0077] The selection of the region of the variable transmission
frequency and the consequential necessary step-up ratio will
therefore be based on a low enough frequency to have a stabile
transmission for the given step-out distance and keep the Ferranti
effect and skin effect low combined with a suitable number of poles
and torque of the motor and the generator. Additionally, if oil
filled motor and generator are preferred, the speed must be kept
below some limit to avoid too high frictional losses; typically
could a speed of 750 to 1500 rpm be favourable, i.e. a transmission
of 25 Hz to obtain 750 Hz for 4-pole motor and 1500 rpm for 2-pole
motor.
[0078] Below is given as an example a table that shows the
resulting speed of a subsea compressor drive (motor) with 2-poles
by using a motor-generator set with 4-poles motor and 12-poles
generator:
TABLE-US-00002 Transmission Output frequency Speed of 2-poles
frequency, Speed of 4-poles from 12-poles compressor drive, Hz
motor, rpm generator, Hz rpm 5 150 15 900 10 300 30 1800 20 600 60
3600 25 750 75 4500 30 900 90 5400 40 1200 120 7200 50 1500 150
9000 60 1800 180 10800 70 2100 210 12600 80 2400 240 14400
[0079] The table demonstrates that a transmission frequency range
up to 50 Hz will cover the most actual speed range for
compressors.
[0080] A similar table is given below for a compressor drive with
2-poles, a 6-poles motor for the motor-generator set and 24-poles
generator:
TABLE-US-00003 Transmission Output frequency Speed of 2-poles
frequency, Speed of 6-poles from 24-poles compressor drive, Hz
motor, rpm generator, Hz rpm 1 20 4 240 5 100 20 1200 10 200 40
2400 20 400 80 4800 25 500 100 6000 30 600 120 7200 40 800 160 9600
50 1000 200 12000 60 1200 240 14400 70 1400 280 16800
[0081] In this case a transmission frequency of up to 40 Hz will be
sufficient.
[0082] The above tables clearly demonstrate that the transmission
frequency can be kept low to avoid problems caused by Ferranti
effect and skin effect.
[0083] Selection of compressor bundle is also a factor that helps
to give freedom in selection of transmission frequency and
frequency step-up ratio, i.e. a bundle can be selected, within
reasonable limits, to suit an f.sub.s-u resulting from an optimum
transmission system.
[0084] A subsea RFSD is in principle quite simple and no control
system is need because the stepped-up frequency will be
automatically obtained as a result of the ratio of poles of the
generator relative to the poles of the motor of the RFSD.
[0085] Another advantage with a subsea rotating step-up device is
that the output current and voltage will have a practically perfect
sine wave form which is beneficial for the motors, i.e. no electric
filter for smoothening is needed to obtain this.
[0086] The subsea RFSD (SRFSD) also supplies inductance to the
transmission system, which due to the cable has a surplus of
capacitance, and the SRFSD therefore reduce the need for near end
electric phase compensation.
[0087] There will be some power loss in a SRFSD, say 5%, but a
subsea VSD will also have losses, however perhaps lower.
[0088] The selection of SRFSD must of course be such that the
output power of the generator at a given frequency is such that it
corresponds to the demand of the connected motor(s). If for
instance a 2-pole compressor motor shall give 10 MW at 10000 rpm,
the power output of the generator must be accordingly plus a little
additionally to cover for losses at a frequency of 167 Hz. The
motor of the SRSFD must correspondingly give a shaft power of 10 MW
plus some additionally to cover for losses.
[0089] Another way than having different poles of the motor and
generator of the motor-generator set, can be to include a fixed
step up gear between the motor and the generator, e.g. of 3:1. If
the transmission frequency for instance is 50 Hz, a 4-poles motor
will have a speed of 1500 and the generator speed will be 4500 rpm
with an output frequency of 150 Hz that gives a 2-poles compressor
drive a speed of 9000 rpm. A combination of fixed step-up and
number of generator poles can also be used to keep the number of
poles down if favourable. If for example a step-up gear with ratio
2:1 is inserted between a 4-poles motor and an 8-poles generator,
the speed of the motor at 50 Hz will be 1500 rpm, the speed of the
generator 3000 rpm and its frequency output 200 Hz and the speed of
the drive 112000 rpm. By having VSD at the near end the speed of
the drive can be adjusted to suitable values by adjusting the
transmission frequency in the range up to 50 Hz.
[0090] In some cases can be kept a fixed transmission frequency and
thereby a fixed frequency from the generator and hence a fixed
speed of the connected motor, e.g. compressor, multiphase or single
phase pump motor. If the motor runs a compressor, the compressor
speed can for instance be kept constant at 9000 rpm, and a suitable
flow capacity and pressure ratio of the compressor, which will vary
over time, can be adjusted by rebundling and some recirculation.
This will give the simplest and lowest CAPEX of the total system,
but with somewhat higher power losses due to periods with
recirculation on the compressor. A more frequent rebundling of the
compressor may also be necessary compared to variable frequency. An
optimum power transmission and compression system must be based on
calculations to establish optimum system design from case to
case.
Design of Subsea RFSD
Oil Filled Pressure Housing
[0091] The motor and generator are assembled in a common pressure
housing with a suitable number of flanges with seals. Further there
are several options for the practical design, which are listed in
the following:
[0092] The motor-generator has a suitable number of bearings.
[0093] The rotational speed of the motor-generator is low enough to
keep the frictional losses acceptable, and the common pressure
housing is filled with a suitable liquid, e.g. oil, that lubricates
the bearings and also cools motor and generator and the properties
of the selected oil should preferably be such that it serves as
electric insulator.
[0094] Instead of oil, the housing can be water filled with water
or a mix of water and antifreeze agent, e.g. MEG, which requires a
complete electrical insulation of the motor and generator
windings.
[0095] The pressure inside the housing can be selected freely by
not filling it completely with liquid and have a gas volume at some
pressure.
[0096] A favourable solution is to fill the housing with liquid and
have pressure balancing device between the ambient seawater and the
internal liquid of the pressure housing. This will result in a
minimum thickness of the pressure housing and also reduce the load
and requirements to flanges and seals
[0097] If the direct cooling of the motor-generator by heat flow
through the pressure housing and to the sea is too low, has to be
included an external cooling circuit with heat exchange to the
ambient seawater.
[0098] The pump for the cooling circuit can favourably be coupled
to the motor-generator shaft or it can be a separate pump with
electric motor.
[0099] If magnetic bearings for operation in liquid are available,
this could be an option to liquid lubricated bearings. For more
details about this, reference is made to the description below for
gas filled housing.
Gas Filled Housing
[0100] The pressure housing can be filled with an inert gas, e.g.
dry nitrogen or dry air.
[0101] The advantage of this is lower frictional losses than for
oil filled, which allows higher speed of motor-generator.
Additionally the practical solution can include the following:
[0102] Liquid lubricated bearings (e.g. oil, water or water/MEG)
with a circulating circuit through an external heat exchanger or
only inside the housing.
[0103] Minimum one pump for the lubricant, either driven by the
motor-generator shaft or a separate electric pump
[0104] If necessary a cooling circuit for the gas is included by
having minimum one fan to circulate the gas through an external
heat exchanger or only inside the housing.
[0105] Alternatively to liquid lubricated bearings, magnetic
bearings can be used. The cooling system for the gas must then be
dimensioned to also cool the magnetic bearings.
[0106] A control system for the magnetic bearings must be included,
located in the vicinity of the motor-generator housing or inside
the housing. If the control system is located in a pod outside the
motor-generator housing, penetrators through the housing wall are
needed as well as wires for power and signals between the control
system and the magnetic bearings. If the control system is in a
pod, the pod can be designed to be separately retrievable or
not.
[0107] The pressure inside the housing can be selected from in the
region of one bar and up to equal to the ambient water pressure or
higher. The advantage of low pressure is low friction and losses.
The advantage of high pressure is that the heat capacity of the gas
increases with pressure and therefore gives better cooling. Another
advantage of high pressure is also reduced requirement to wall
thickness and lower load on flanges and seals. If the pressure is
selected close to equal to ambient seawater pressure, the resulting
requirements to the pressure housing and flanges and seals will be
similar to a liquid filled pressure balanced vessel.
Subsea Rotating VSD
[0108] Above is mentioned use of hydraulic or fluid coupling
between the motor and the generator in the motor-generator set.
Such a coupling has the advantage of giving "soft start", i.e. the
generator load on the motor is not immediate, but ramps up over
some time such that a high start current peak is avoided. The use
of such a coupling can be further expanded to make the coupling
adjustable such that the speed of the generator can be adjusted
relative to the constant motor speed. In this way the
motor-generator set can be used as a subsea variable speed drive,
i.e. subsea rotating variable speed drive (RVSD), and the topside
VSD can be omitted.
[0109] Instead of a fluid coupling can be used a mechanical gear
for stepping up and down the speed of the generator, and thereby
its output frequency.
[0110] If a variable coupling of some kind (fluid or mechanical) is
used, the control system for the variable coupling can be in a
separate pod externally to the subsea RVSD or it can preferably be
surface located and preferably connected to or integrated in the
overall control system for the subsea booster station, compressor
station or subsea processing plant or other system with subsea
motors with variable speed.
Some Considerations
[0111] One important point of the invention is that though
typically a VSD is used at near end, it is not important to be able
to quickly adjust the frequency of the motor loads. The motor speed
is slowly adjusted over years while the reservoir is produced and
the field pressure gradually decreases thus requiring increased
power, i.e. motor speed. This fact allows for e.g. temporarily
ramping down running motors in order to connect one more motor.
Alternatively, the unused motor can be connected direct on load if
calculations have demonstrated that this is feasible with respect
to current peaks or other disturbances of the power transmission
system. Depending on the number of already running motors it can be
beneficial to temporarily reduce the frequency before the
DOL(direct on-line) start. If necessary the power can be switched
off when starting an additional motor and then start and ramp up
the speed of all motors simultaneously. In a compression station
another option is to put all pumps and compressors in recirculation
before starting up a compressor or a pump that has been stopped,
then start the stopped unit and when it has reached the desired
speed, put all compressors and pumps on line in production
mode.
[0112] The above mentioned devices and methods make it possible to
manage the Ferranti effect and skin effect and thereby considerably
extend the distance for stable subsea high-voltage power
transmission.
[0113] Hence maximum practical step-out distance can be very much
increased without introducing subsea VSDs with local subsea control
of the frequency.
[0114] Both in FIGS. 4 and 5 the step-up devices have not a local
control system that varies the frequency and thereby the speed of
motors according to the production, neither do they directly
control the ramping down of frequency to add operation of motors
that have been stop nor do they directly control the ramping up of
the frequency to obtain the actual speed of the motors to suit the
production.
[0115] If the RFSD has oil lubricated bearings, there is no need
for any control system of the unit, and possible instrumentation
can be limited to monitoring, e.g. vibrations and temperature, if
found beneficial.
[0116] As mentioned in the section: "Background of the invention
and prior art" the speed of compressors can typically range from
e.g. 4000 to 14000 rpm and of pumps from e.g. 2000 to 5000 rpm.
When compressor and pump motors in a compression station according
to the invention (Fourth and Fifth Solution) are supplied with the
same frequency by a common transmission cable, the speed of the
pumps can easily be adjusted to the desired speed of half of the
compressor speed by using four-pole or more pole motors for the
pumps and two-poles motors for the compressors. If the pumps are
used for controlling the liquid level of a separator in a
compressor station, a suitable variable net forward flow for the
pump can be arranged by re-circulation and equipped with flow
control valves.
[0117] The speed of the pumps can therefore be controlled in the
following optional ways:
[0118] Dedicated subsea FSD for each pump motor
[0119] One common FSD for several pumps motor
[0120] Running the pump motors on same frequency as the
compressors, but with the double number of pole resulting in half
the rotational speed
[0121] Running the pumps on the transmission frequency whilst the
compressors power frequency is stepped up.
[0122] Generally, for the number of subsea FSDs, their number can
be from one per motor to one big common unit for all motors or
something in between, e.g. one FSD per large compressor motor and
one common unit for the quite small pump motors or, as mentioned
above, no FSD for the pump motors.
Some Suggested Combinations of Surface Located VSDs, Number of
Subsea Drives and Number of 3-Phase Transmission Line
[0123] A 3-phase transmission line consists of three individual
cables that are insulated and bundled together. For long subsea
transmission with more than one motor, e.g. two compressors, it is
with present technology possible to bundle together transmission
lines for two motors, i.e. six cables in the bundle. This will
reduce the laying cost of the lines and has the advantage of
allowing individual frequency control of two motors at the far end
of the two lines that are bundled together. There is one step-up
device per motor. Such an arrangement is shown in FIG. 7. In this
case the motor is of the high voltage type and the transmission
voltage can be e.g. 100 kV and there is no need for subsea
transformers. In such case the circuit breaker has to be located
after the generator where the voltage is acceptable because subsea
circuit breakers for very high voltages like 100 kV are presently
not available.
[0124] Another way, which results in lower investment, is the
solution shown in FIG. 4 and with a hydraulic soft starter between
motor M and generator G such that the motors M1-M4 can be started
individually without unacceptable start currents. All motors will
operate at same speed, which is not a problem for equal machines,
e.g. compressors.
[0125] The less complicated arrangement is that of FIG. 4 without
soft starter. In this case it will be necessary to start all
compressors simultaneously, and this is a little inconvenient but
not considered a problem because number of start-ups per year is
limited.
[0126] In Table 2 is explained the meaning of the items in the
figures.
TABLE-US-00004 TABLE 2 Figure labels. Item # Explanation 1 Electric
power supply grid 2, 2', 2'', 2''' Step-down transformer 3, 3',
3'', 3''' VSD, Variable Speed Drive 4, 4', 4'' 4''' Step-up
transformer 5, 5', 5'', 5''' Transmission cable 6, 6', 6'', 6'''
Step-down transformer 7, 7', 7'', 7''' Circuit breaker 8, 8', 8'',
78''' Near end of transmission cable 9, 9', 9'', 9''' Far end of
transmission cable 10 Common bundle of two or more power
transmission lines 11 Pressure housing 12 Inert gas or liquid 13,
13', 13'', 13''' Step-down transformer 14, 14'', 14'', 14''' VSD
15, 15', 15'', 15''' Circuit breaker 16, 16', 16'', 16''' Rectifier
17 Fluid (hydraulic) coupling (optional), stepless fluid gear
(optional) or fixed ratio gear box (optional) 18, 18' Penetrator 19
Pressure balancing unit M1, M2, M3, M4 Motor M Motor of the subsea
rotating frequency step-up device (subsea RFSD) G Generator of the
subsea rotating frequency step-up device
DETAILED DESCRIPTION
[0127] Reference is made to FIG. 4, illustrating a specific
embodiment of the present invention. Node 1 is connected to a
source for electric power; the source is a local power grid or, for
instance, a local power generation system. A VSD 3 is connection to
power source. A VSD input transformer 2 is often connected in
between in order to adjust the supply voltage, e.g. 13.8 kV for a
platform to the rated VSD voltage, e.g. 6 kV. The transformer can
be an integrated part of the VSD as offered by some suppliers.
Normally a step up transformer 4 is needed to connect the VSD 3 to
the high-voltage transmission line 5 that in the example of a
subsea application consists of a cable. A typical voltage applied
to the cable could for instance be about 120 kV. The cable is laid
into the sea in order to extend from the near end 8 to the subsea
far end 9; the cable has any operative length where the Ferranti
effect starts being observed until where it strongly dominates to
the load current. This can be translated to length in the order 20
km, to 100 km and probably beyond, dictated by the location and
properties of the subsea loads. At the far end 9 of the cable, a
subsea transformer 6 is arranged, stepping down the voltage to for
example 20 kV suitable for the circuit breakers 7, 7', 7'', 7''',
followed by transformer 13, 13', 13'', 13''' stepping down to for
example 6 kV suitable for the motors of subsea RFSDs or the
operational voltage of SFSDs, which is also a suitable voltage for
the motors M1, M2, M3, M4. Four subsea motors are illustrated,
which for instance could be two compressor motors M1, M2 and two
pump motors M3, M4.
[0128] The step down transformers are in principle optional because
the step-down transformer 6 (ref. FIGS. 4 and 5) can directly
step-down the voltage suitable for the subsea FSDs as illustrated
in FIG. 5. Inclusion of 13, 13', 13'', and 13''' is a question of
optimisation of the far end power distribution system.
[0129] The subsea RFSDs in FIGS. 4 and 5 step up the transmission
frequency with a desired step up ratio by selection of poles of the
motor M and Generator G.
[0130] It shall be emphasised that the key components of the power
transmission systems of FIGS. 4 and 5 are the power source 1, the
variable speed drive (VSD) 3, the transmission cable 5 and the
motor-generator set M-G. The other components, i.e. step-up and
step-down transformers, 2, 4, 6, and 14, 13', 13'', 13''', and
circuit breakers 15, 7, 7', 7'', 7''', are included according to
need from case to case.
[0131] If for instance the motor M of the motor-generator set is of
the type with insulated cables in the stator it can operate at a
much higher voltage than motors with conventional coils. Hence both
the step down transformers 4, 6 and 13 may become superfluous. If
additionally the motors M1-M4 are run at fixed sped from the
step-up devices, the VSD 3 can be omitted.
[0132] Another advantage of high voltage subsea motors with
insulated stator cables is that they need less current (amperes)
through the penetrators through the motor housing than motors at
conventional voltage in the range of 6 kV. This will allow for
motors with higher power than at the present stage where around 12
MW is the maximum due the limitation in current (ampere)
capacity.
[0133] Cost of long subsea cables and subsea VSDs is high, and
subsea VSDs in FIG. 2 have a negative impact on system reliability
as well as being expensive. One common transmission cable compared
to the solution in FIG. 1 therefore represents a considerable
saving in investment.
[0134] It shall be mentioned that even though one common
transmission cable is beneficial of cost reasons, there is
technically no problem to have one transmission cable for each
subsea FSD. This may be the optimum solution for medium step out
lengths, say 35 to 75 km, i.e. up to distances where the cable cost
does not become prohibitive. With one VSD per transmission cable,
i.e. one VSD per subsea motor, this results in individual speed
control for each motor.
Condensed Description of the Invention Subsea Step-Up Device
[0135] It is problematic or even not possible to transmit high
voltage high power electricity at high frequency, say more than 100
Hz, over long subsea step-up distances, say more than 40 km, to
supply motors operation at high speed for subsea pumps and
compressors. This is due to the Ferranti effect that can create
over voltage and instability in the transmission system as well as
the skin effect that creates high ohmic resistance and consequently
high voltage and power losses.
[0136] Subsea variable speed drives to which the transmission
frequency can be low, e.g. 50 Hz, presents a solution to this. They
are however big and equipped with a large amount of sensitive,
fragile electric and electronic components and control system,
which additionally to making them expensive also are assumed to
have a high failure rate.
[0137] The invention offers a solution to this by having the VSD
with its control system at surface (on a platform or onshore) and
then having one or more simple rotating subsea frequency step-up
devices, near the subsea motors. These devices preferably do not
directly control the frequency of the electric current to the
motors, their only function is to step up the transmission
frequency, which is variable and set at frequency according to the
need of the motors, by a suitable ratio. In the case of rotation
subsea frequency step-up devices, the resulting step-up ratio is
resulting from the ratio of numbers of poles of the generator and
the motor of the device. The ratio will for instance be 2 if the
generator is 4-pole and the motor 2-pole.
[0138] As stated above, the preferred function of a SRFSD is purely
to step-up the transmission frequency, and the variation in the
output frequency is decided by a near end surface located VSD. The
exception from this is if there is no VSD or similar control device
at the near end. In such cases the output frequency of the SRFSD
generator can be fixed or be varied within some limits by including
some type of adjustable coupling or gear (e.g. a mechanical gear, a
fluid-dynamic or hydraulic gear, a mechanical fluid dynamic gear or
a magnetic gear) between the motor and the generator of the
SRFSD.
[0139] Rotating subsea step-up devices add inductance to the
transmission system and are therefore beneficial by counteracting
the large capacitance of the cable, and therefore the near end
compensation system can probably be reduced.
* * * * *